A heat-sensitive recording system is widely known in which a thermal head is allowed to scan in close contact with the surface of a heat-sensitive recording material comprising a heat-sensitive coloring layer on a support so that heat energy is transferred to the heat-sensitive coloring layer directly or through a protective layer to record a color image thereon. This heat-sensitive recording system is employed in a facsimile, printer, etc. However, since the thermal head is allowed to scan in close contact with the heat-sensitive recording material, the thermal head wears out or tailings of the components of the heat-sensitive recording material are attached to the surface of the thermal head, causing the problem that correct recorded images cannot be obtained or the thermal head is destroyed.
Furthermore, the heat-sensitive recording system using such a thermal head is disadvantageous in that the structural properties of the thermal head restrict the high speed control of heating and cooling of the heating elements and the density of the heating elements, giving a limit to the speed, density and quality of recording.
In order to eliminate the above-mentioned difficulties of a heat-sensitive recording system using a thermal head, a system in which a laser beam is used to effect recording at a high speed and a high density in such a manner that the thermal head is not brought into contact with the heat-sensitive recording material has been proposed, as disclosed in JP-A-50-23617, JP-A-54-121140, JP-A-57-11090, JP-A-57-14095, JP-A-57-14096, JP-A-58-56890, JP-A-58-94494, JP-A-58-134791, JP-A-58-145493, JP-A-59-89192, JP-A-60-205182, and JP-A-62-56195 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, such a recording system using a laser beam is disadvantageous in that since a heat-sensitive coloring layer generally can hardly absorb visible light and near infrared rays, the laser power must be considerably great to obtain the heat energy required for coloring, making it extremely difficult to provide a small-sized and inexpensive apparatus.
JP-B-50-744 (term "JP-B" as used herein means an "examined Japanese patent publication") proposes a system in which a paper coated with microcapsules containing an ink is irradiated with a strong light so that the ink is released from the microcapsules to effect recording thereon. However, this system exhibits a very low sensitivity and thus is not yet realized.
Many proposals have been made to allow the heat-sensitive coloring layer to efficiently absorb a laser beam. In most cases, a light-absorbing substance that fits to the wavelength of a laser beam used is incorporated in the heat-sensitive coloring layer. In this case, if the light-absorbing substance to be used is not white, the background of the recording material is colored, giving a low contrast and unrefined record. Further, if the light-absorbing substance added is not present in the portion requiring direct heating, a recording material with a reduced sensitivity is provided.
In general, colorless or white light-absorbing substances occur mostly in the form of inorganic compounds. However, most of these inorganic compounds exhibit a low light absorption efficiency., Accordingly, even if the light absorption efficiency itself is improved by incorporating a light-absorbing substance, if the heat-sensitive coloring layer is opaque, laser beam is partially reflected, and the amount of laser beam absorbed by the heat-sensitive layer is reduced, making it impossible to record images. Therefore, it has been desired to develop an organic compound with an excellent light absorption efficiency which is little colored. Further, it has been desired to obatin a transparent recording layer for the purpose to form a transmitted image and a multicolored image by a multilayer coating.
In recent years, semiconductor lasers have shown a remarkable progress, and small-sized and inexpensive semiconductor lasers have been available. However, since the oscillation wavelength of these semiconductor laser is in the near infrared range, it has been desired to develop a heat-sensitive recording material which can absorb light in the near infrared range to effect heat recording.
However, organic compounds which absorb visible light are normally colored. The darker these organic compounds are colored, the higher is their light absorption efficiency. Therefore, the sensitivity of the heat-sensitive recording material can be increased by incorporating these organic compounds in the heat-sensitive recording layer (hereinafter referred to as "heat-sensitive layer") as a light-absorbing substance. However, it is difficult to improve the whiteness of the recording paper by using these organic compounds.
The heat sensitivity of the recording material can be improved by increasing the added amount of the light-absorbing substance. However, since the recording material itself is colored dark in proportion to the added amount of the light-absorbing substance, it is difficult to provide an excellent recorded image.
However, colored organic compounds with an excellent efficiency of absorbing a laser beam, if they can render themselves colorless after recording by a laser beam, can render the heat-sensitive layer in the recording material colorless and thus render the background of the recording material white or colorless after image recording, enabling the improvement in the heat sensitivity by the increase in the content of these organic compounds while improving the image quality.
On the other hand, if an organic compound which does not absorb visible light but absorbs a laser beam of a wavelength outside the visible range is used, the background of the recording material can be rendered white even if it is incorporated in the heat-sensitive layer because it is not colored. However, the kind of these organic compounds which can be added is limited.
U.S. Pat. No. 4,816,367 proposes a multicolor imaging material in which heat-meltable micorcapsules containing a color former and an infrared absorbent in the wall or inside of the microcapsules and a developer exist in the same layer or different layers. Also, U.S. Pat. No. 4,916,042 process a multicolor imaging material comprising a support having provided thereon a diazo compound, a coloring assistant and heat-meatable microcapsules having a porous membrane which encloses the coupling component and a capsule wall which includes the infrared absorbent. However, since the infrared absorbent in contained in the wall or inside of the microcapsules, it is difficult to incorporate a large enough amount of the infrared absorbent in the microcapsules, making it difficult to obtain a laser power required for providing an excellent recorded image. Further, since the capsule wall is heat-meltable, the storage stability is poor. In these U.S. patents, there is no disclosure with respect to a method of providing a heat-sensitive recording material for an infrared laser with an excellent transparency.
Also, U.S. Pat. No. 5,198,321 proposes an image forming method comprising bringing a transparent heat-sensitive recording material into contact with an infrared-absorbing layer, irradiating with a laser beam to recored an image, and separating the infrared-absorbing layer and the heat sensitive recording material from each other. However, since the heat-developed portion and colored portion are different layers, it is difficult to obtain a sufficient sensitivity. Further, this method suffers from the defect that the heat-developed portion needs to be disposed as waste.
As a result of extensive studies to obtain a highly refined heat-sensitive recording material with a nearly white background capable of non-contact recording, the inventors found that a heat-sensitive layer formed by the coating of a coating solution containing an emulsion obtained by the emulsion dispersion of a solution of a coloring component and an infrared-absorbing dye in an organic solvent slightly soluble or insoluble in water exhibits an excellent transparency and thus can efficiently absorb infrared rays without scattering, enabling excellent image recording with an infrared laser beam while rendering the background of the heat-sensitive recording material almost white. Thus, the present invention was developed.